Extended electron states in lateral quantum dot molecules investigated with photoluminescence

InAs quantum dot molecules (QDMs) formed by molecular-beam epitaxy on GaAs (311)B substrates through self-organized anisotropic strain engineering are studied by excitation-power-density- and temperature-dependent macro- and microphotoluminescence (PL). An unusual asymmetric broadening, together with a continuous shift toward higher energies of the PL peak position, most prominent for the p-type modulation-doped QDMs, is observed with increasing excitation power density. The n-type modulation-doped QDMs exhibit a square-shaped PL spectrum, resembling that of modulation-doped quantum wells. In temperature-dependent macro-PL, two distinct minima of the full width at half maximum are observed, indicating thermally activated carrier redistribution within the QDMs through two different channels at lower and higher temperatures. The micro-PL spectra of the p-type modulation-doped QDMs exhibit discrete sets of sharp peaks on top of broad PL bands. The number and intensity of the sharp peaks increase with excitation power density. With increasing temperature, the number and intensity of the sharp peaks decrease while the intensity of the broad PL bands increases, in agreement with the carrier redistribution at lower temperatures. Only broad PL bands are observed for the n-type modulation-doped QDMs with similar behavior. These results are explained by state filling in the presence of extended electron states formed due to lateral electronic coupling of the quantum dots within the QDMs.